1. Field of the Invention
The present invention relates to a communication mediation apparatus and a method for controlling the same, and more particularly to an apparatus and method for performing communication mediation while reducing a communication miss rate caused by collision of data.
2. Description of the Related Art
There have been widely used systems for recognizing a plurality of moving objects by allowing a single controller to communicate with transponders of the moving objects, for example, an automatic charging system, a system for managing domestic animals, and a system for managing stored goods, etc.
There have been proposed a variety of communication mediation methods for performing communication mediation to reduce a communication miss rate caused by collision of data in the above-identified systems, for example, a mediation algorithm for adapting a unique number assigned to each transponder, another mediation algorithm for adapting a random delay count value, and a persistence-mediation algorithm, etc.
The mediation algorithm for use with the random delay count value and the persistence-mediation algorithm will hereinafter be described in detail with reference to vehicles and roadside controllers (RS-Cs).
The mediation algorithm for use with the random delay count value uses a random number generator or counter built in an in-vehicle device such as a transponder in such a way that it solves a data collision problem caused when more than two vehicles generate response signals at the same time. In more detail, in the case where a transponder transmits a response packet to reply to a control signal received from the roadside controller, and no acknowledge signal is then received from the roadside controller within a predetermined time (i.e., if it is determined that data collision occurs), the mediation algorithm for use with the random delay count value generates random numbers according to a mediation parameter (e.g., a maximum count value) contained in the control signal received from the roadside controller, counts the random numbers to create a delayed time, and then retransmits a response packet.
FIG. 1 is a conceptual diagram illustrating a mediation algorithm for use with such a random delay count value.
A roadside controller transmits a control signal for every downlink timeslot “b” within a communicable range. In the case where each uplink timeslot “a” receives an accurate response signal having no error, i.e., in the case where communication interferences or response signals simultaneously created by many vehicles are not generated, the next downlink timeslot transmits an acknowledge (ACK) signal. The reason why the next downlink timeslot transmits the ACK signal is to allow a vehicle sending the above response signal not to answer the next control signal.
Vehicles, i.e., a vehicle A, a vehicle B, a vehicle C, a vehicle D, and a vehicle F, enter their communicable ranges, respectively, and receive control signals from the roadside controller (not shown) in downlink timeslots 101, 109 and 113. The vehicles A, B, C, D and F transmit response signals to the roadside controller in uplink timeslots 103, 111 and 115. Black-colored dots shown in the uplink timeslots 103, 111 and 115 represent the fact that vehicles transmit their response signals.
Referring to FIG. 1, the vehicles A, B and C enter a communicable range at the same time, such that data collision occurs among response signals of the vehicles as shown in a reference number 103 of FIG. 1. Therefore, the roadside controller receives an erroneous response signal so that The ACK signal is not transmitted in the downlink timeslot 105.
Because the vehicles A, B and C do not receive the ACK signal within a predetermined time denoted by a timeslot 103, the vehicles A, B and C generate random numbers according to a mediation parameter (i.e., a maximum count value) contained in the received control signal, count the random numbers to create a delayed time, and then retransmit response signals.
The vehicle A retransmits a response signal in the uplink timeslot 107 by referring to the above time delay result. Because vehicles other than the vehicle A do not transmit response signals in the timeslot 107, the roadside controller receives a response signal having no error. But, not only vehicles B and C but also a vehicle F entered from the timeslot 113 transmit response signals in the uplink timeslot 115 at the same time, resulting in data collision in the uplink timeslot 115.
As stated above, if vehicles do not receive ACK signals within a predetermined time after transmitting their response signals, the vehicles create a delay time using a mediation parameter (i.e., a maximum count value) contained in a control signal, and then repeatedly retransmit the response signal. As a result, the vehicle B successfully transmits a response signal in an uplink timeslot 127, and the vehicle F successfully transmits a response signal in an uplink timeslot 131.
On the other hand, FIG. 1 shows a random delay count value algorithm for enabling a vehicle to immediately transmit a response signal when the vehicle receives an initial control signal. There has also been proposed another random delay count value algorithm for creating random numbers after a vehicle receives an initial control signal, creating a delay time by allowing the vehicle to wait for timeslots corresponding to the created random numbers, and allowing the vehicle to transmit a response signal.
The persistence-mediation algorithm determines whether a vehicle receiving a control signal can transmit a response signal on the basis of the value of a created random number. For example, a vehicle receiving a control signal creates random numbers from 0 to 999. If the created random number is beyond a predetermined random number 500, the vehicle waits for the next control signal. If the created random number is lower than the predetermined random number 500, the vehicle immediately transmits a response signal. In this case, the predetermined random number 500 functions as a threshold value needed to determine whether the vehicle transmits the response signal, and is also called a persistence value (i.e., a mediation parameter).
The above-identified communication mediation algorithm is characterized in that successful communication is performed on the condition that only one moving object transmits a response signal in one uplink timeslot, and data collision in communication occurs on the condition that more than two moving objects transmit response signals. Therefore, it is necessary for an appropriate communication mediation algorithm to allow all the moving objects (i.e., transponders) to communicate with a controller within a communicable range in consideration of data collision in communication.
However, a performance (i.e., a communication miss rate) of a conventional communication mediation method for use in the above-identified system for recognizing moving objects varies with the number of moving objects (i.e., traffic volume variation), resulting in deterioration of system reliability.